It depends on a few variants.
What state are you in?
The ratio in Qld is currently lower than other states, we have 1:12 for 3-5 year olds. When you have school aged children in there, if anyone is under 4, the ratio drops back to 1:7 in Qld. Either way, what they have is not a legal ratio.

So the Qld ratios are:
1:4 up to 15 mths of age
1:5 15mths - 2 years
1:6 2-2.5 years
1:8 2.5-3 years
1:12 3-5 years
1:7 for mixed age grouping which has a number of different grouping mixes allowed.


Also - legally centres should display what group they are running on the door of the room at all times so you should be able to see what ratio they should have in your state.